Download The benefits of manufacturing topsoil using PAS100 compost

Information document for the construction industry
How manufacturing topsoil using
PAS100 compost can reduce costs
for the construction industry
This leaflet presents the results of a recent trial using PAS100 compost to
manufacture soil for use in the construction industry. It also provides good
practice for using it.
Summary
 Construction sites often suffer from a lack
of topsoil for landscaping.
 Certified compost can be readily sourced
and imported on to site to manufacture
topsoil.
All compost certified under the Compost
Certification Scheme must meet the quality
requirements of the PAS100 specification,
ensuring its quality and allowing it to be used
for landscaping without any need for
environmental permits.
 Manufactured
topsoil can out-perform
imported ‘natural’ topsoil in terms of grass
growth and soil development.
 The cost of on-site topsoil manufacture is
usually considerably less than importing
topsoil.
 The process of manufacturing topsoil on
site should be overseen by a landscape
architect, landscape contractor or soils
consultant, to ensure the specific
landscaping soil requirements are met.
What is compost?
Compost is the result of the controlled
breakdown of biodegradable material, under
aerobic conditions.
There are two main types of compost: green
and green/food compost. Green compost is
solely derived from garden waste from
sources such as domestic gardens and
municipal parks. Green/food compost is made
from similar inputs, but with the addition of
food waste, such as that collected from
households. Both kinds of compost are
excellent soil conditioners, the main difference
between them being the rate they release
nutrients, which tends to be higher in
green/food compost.
Using compost to make topsoil
Certified compost is already being used
successfully in topsoil manufacturing on
development projects. Topsoil is produced by
mixing compost with recycled inert materials
already present on site, such as surplus subsoil, stone fines and other mineral soil-forming
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Information sheet
materials. The particular blend of certified
compost and soil-forming material will vary
according to the landscaping needs to each
individual site, but all manufactured topsoil
should conform to the requirements of
BS3882:2007.
used for larger-scale operations.
Grass growth trial
The Llanelli School regeneration scheme
offered an excellent case study of the use of
certified compost for large-scale urban
landscaping in the construction industry. A
trial
was
undertaken
comparing
the
performance of amenity and wildflower
grasses established on manufactured and
imported topsoil.
The plots were monitored visually, and grass
production was measured over a period
between July and October to determine the
effectiveness of manufactured soil compared
with imported soil.
One test area used standard imported soil.
The second test area used topsoil that was
manufactured onsite by incorporating PAS100
certified (green) compost at the rate of 10%
by volume within the top 20 to 40 cm of
prepared subsoil. This was achieved by
spreading the compost to a depth of 4 cm
across the subsoil surface and rotavating it in.
A small tractor-mounted rotavator could be
Moisture content of the subsoil during
manufacture is critical. It needs to be friable
to allow the clods to break up easily to
incorporate the compost.
Grass growth results
Laboratory results show the compost had
twice the organic matter content of topsoil,
nearly twice the total nitrogen content, five
times the readily-available nitrogen, and twice
the available potassium and phosphorus.
Consequently, using compost allowed for the
importation of a concentrated form of plant
nutrients onto the site without bringing in the
weight of the mineral matter in topsoil. The
grass growth trials have shown that over a
single season between June and October
Grass growth on the wildflower seed mix plots. Twice the growth is noted on the plots manufactured on-site
with PAS100 compost.
Grass growth on the amenity seed mix plots. Twice the growth is noted on the plots manufactured on-site
with PAS100 compost.
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2014, the topsoil manufactured with certified
compost outperformed the imported soils by a
factor of two. This means it had
approximately 100% more growth than the
imported soils (see table 1 and photos above).
Cost benefit
Considerable variations in costs of topsoil and
compost can be expected from site to site,
and the costing savings below are based on
those relating to the Llanelli School scheme.
Amenity grass seed mix
Type of topsoil
Wildflower grass seed mix
Imported
Manufactured
Imported
Manufactured
113
224
72
160
Dry weight of cut
grass/m2 (g)
Table 1 Grass growth results
Soil development effects
A sharp boundary surface can be created if
imported or manufactured topsoil is simply
placed on the site surface. This can lead to
topsoil slippage on slopes. In addition, root
penetration can be restricted if density is
significantly high.
The cost of importing topsoil would have been
£20 per tonne delivered to site. Spreading this
at 10 cm depth (with 1 tonne covering 10 m2)
would cost £2 per m2.
The compost cost was around £35 per tonne
delivered to site. Spreading it to a depth of
4 cm over 25m2 cost £1.40 per m2.
An additional estimated cost of £0.25 per m2
for compost incorporation was added to the
cost of spreading the imported topsoil.
Based
on
these
estimates,
topsoil
manufacture with certified compost would
cost between 15% and 20% less than
importing topsoil. There are also likely to be
additional medium and long-term benefits
from the added nutrients in the compost.
Soil development. Imported soil
Soil development. On-site manufactured soil
Root penetration in deeper soil results in
faster soil development. This is particularly
important on sites that need soil stabilisation
and access to water reserves, especially when
dry conditions are expected. By incorporating
PAS100 certified compost into manufactured
topsoil, these risks can be reduced.
The results from the trial showed an increased
grass growth in the manufactured topsoil.
However, if this degree of growth is not
required or desirable, you could reduce the
compost rate to less than half of the amount
of imported topsoil used to gain further cost
savings.
Information sheet
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